Dipsticks for checking the level of engine oil and transmission oil in motor vehicles are most typically a single narrow strip of steel with a distal end and a proximate end. The distal end is immersed in the fluid whose level is being checked. The proximate end has a handle by which the dipstick is inserted into and removed from a fluid sump, and a seal which prevents fluid from splashing out of the tube and provides a retaining force for the dipstick. The dipsticks are removably disposed within a guide tube. A top of the guide tube serves as a stop for the dipstick, limiting the travel of the dipstick into the fluid sump.
The distal end has marks indicating a full level and a level at which more oil needs to be added. The distal end extends into the fluid without contacting any positive stop. Locations of the marks are dependent on the nominal values of the length of the guide tube, the relative location of the guide tube to the engine or transmission case with which the guide tube is associated, and the relative location of the case to an oil sump screen. Any variations of these values from their nominal values can produce dipstick readings which do not accurately reflect the true fluid level relative to the sump screen position. The effect of guide tube length variation, dipstick length variation, and guide tube positioning variation on the dipstick are cumulative, causing the positioning relationship between the dipstick and the surface of the oil to be subject to a greater amount of variation than the positioning relationship between the sump screen and the surface of the oil.
One approach to minimizing the potential for variation between the dipstick location and the surface of the oil is to have the position of the distal end of the dipstick controlled by an indicator stop in the oil sump. This eliminates the cumulative variation of the guide tube length variation, the dipstick length variation, and the guide tube positioning variation, and substitutes the variation in distance between the stop and the case. The substitution of one source of error for three reduces the total expected variation.
The use of a stop in the oil sump requires the dipstick to deflect in the axial direction to allow the dipstick handle to be seated. A dipstick designed to flex axially so as to accommodate seating of the dipstick handle is currently employed by the assignee of this invention in production transmissions. The dipstick is formed of spring steel, allowing the dipstick to deflect axially by bowing inside the guide tube as the handle is seated after the dipstick has contacted the stop. Because of the potential for cumulative variation between the stop and the top of the guide tube, the deflection needed in the dipstick to seat the handle may produce forces sufficient to push the handle and seal away from the guide tube.
Providing a dipstick with a sufficiently low axial stiffness can potentially require making the dipstick longer, aggravating underhood packaging difficulties, as well as potentially making insertion and removal of the dipstick more difficult. Also, producing a very low axial stiffness by either making the dipstick either very long or very thin can result in the dipstick being subject to buckling on insertion.